Set top box interconnections


“Set top box” (STB) is a term that can include any type of accessory that may connect to the HDTV.  Common STBs are satellite receivers, cable TV receivers, OTA receivers, DVD players, VCRs, and so on.  In the HDTV era, an STB usually has a large memory where the image is reassembled out of the MPEG-2 data.


Standard DVDs do not have enough capacity for HD.  Blu-ray Disk (BD, which requires a blue laser) is generally the only choice for HD movies.  Most BD players will also play standard DVDs.


Warning:  Older DTV receiver models were often very buggy, cranky units.  Before buying a “bargain” used receiver you should probably see what has been written about it on the AVSforum.



STB video output options

At the present time it is a mistake to buy a TV or STB that does not have an HDMI connector.  HDMI has become the industry standard, but other types of connections still exist.  Any STB you acquire will probably have more than one of these output connectors.  When you buy an HDTV and an STB, try to select units that can connect to each other directly.  Otherwise you will have to pay for a transcoder or a video switch box.


  1. CH3/CH4 output   ATSC output by this method is very rare.  Many converter boxes output NTSC this way.


  1. Composite video   This 1-wire standard, in use for many years, conveys complete video images.  It is designed for NTSC and cannot transport HDTV images.  Phono plugs are standard.


  1. S-video   This 2-wire standard is an improvement over composite video.  But it was designed for NTSC and cannot carry HDTV images.


  1. Component video   This 3-wire standard, originally designed for DVD players, can carry HDTV via three wires with phono plugs.  The three wires carry analog raster (image scanning) signals, either red/green/blue or Y/Pr/Pb.  (Y=intensity, Pr=red-Y, and Pb=blue-Y.)  Some units can handle either color scheme.  You must verify that both units can use the same scheme.  Neither scheme is better than the other.  Phono plugs are standard.


  1. VGA   This 5-wire standard, originally devised for computer monitors, carries HDTV raster signals, usually red, green, blue, Hsync, and Vsync.  However in some units Y, Pr, and Pb can substitute for the colors.  Usually the 5 wires are bundled into a single cable.  Five separate cables are advised for runs longer than 12 feet.  The connector can be either a 15-pin VGA connector or five BNC connectors.  Warning: some HDTVs have VGA inputs that only accept computer formats, such as 600x800 and 720x1024.  Many makers use the term RGB in place of VGA despite the confusion that causes.


  1. DVI  (Digital Visual Interface)   This connector conveys HDTV raster-like signals in binary data form.  The data rate is very high (typically 1.65 Gb/s).  Binary data is preferred by monitors that are not CRTs, such as plasma, LCD, DLP, and others.  It also has a control line (DDC) that allows the STB to sense the monitor’s state and native formats.


  1. HDMI   This miniature connector replaces DVI.  It is backward compatible with DVI, and an adapter will connect it to a DVI unit.  It has 19 pins and carries DVI plus digital audio.  It also has another control line (CEC) that lets units control each other.  Cables longer than 15 feet are possible but require special care and possibly special repeater boxes.


  1. IEEE 1394   Also called Firewire or i.link, this is a high-speed serial bus common in computers.   IEEE 1394 is fast enough to carry compressed MPEG-2 video data plus audio and controls.  There is an encryption standard for IEEE 1394, called DTCP (Digital Transmission Content Protection, also called 5C copy protection).  But since IEEE 1394 is an open standard, Hollywood has less control over it.  Since it is a two-way bus, it could allow units to control each other.  IEEE 1394 is just a connector definition plus a software shell.  Additional software is required for the units to talk to each other.  HAVi (Home Audio Video Interoperability) is such software.  HAVi allows plug-and-play recognition of devices, interoperability, and brand independence.


1394 has important technical advantages.  All outboard recording devices will likely use 1394, but Hollywood does not want to allow outboard (dismountable) media.  Some companies have developed radio schemes for moving audio/video material around a home wirelessly.  But these schemes usually require 1394 and might become useless if 1394 is blocked.


  1. USB   Mainly camcorders use this.  Its workings are a lot like IEEE 1394.





For options 1-3 above: If the STB has any of these connectors, it is only for standard definition images.  When a high-def program is being received, these connectors are either disabled or carry an image that has been down-converted to NTSC.


VGA and component video are sometimes called “analog” because they carry signals that are not binary.  DVI, HDMI, and 1394 are sometimes called “digital” connections.  Analog connections are inferior to digital connections when the monitor is not a CRT, but this inferiority can be insignificant if the cables are chosen with sufficient care.  Neither VGA nor component video is superior to the other.  For a cable length of six feet, VGA is more convenient.  For longer runs, component video is usually more convenient.  Component video is the prevalent analog output, and VGA is becoming rare.


Very few displays will draw both 1080i and 720p.  If you feed the set a format that it cannot draw, you will get either a blank screen or garbage.  (The law requires a set to receive all 18 modes.  But the law only regulates tuners, not these intermediate inputs.)  An exception to this is fixed-pixel displays that will re-digitize component video.



DVI and HDMI come with a decryption option called HDCP (High-bandwidth Digital Content Protection), which will decode encrypted programs such as first-run movies.  The motion picture industry requires distributors (HBO, etc) to use HDCP encryption on all high-def movies.  HDCP decryption hardware is proprietary, and any hardware manufacturer must sign a contract in order to include it in his product.  That contract can forbid hi-def analog output (VGA or component video) when encryption is enabled, and allows HDCP decryption to take place only in the monitor.  This is an attempt by Hollywood to prevent unauthorized copying and distribution of hi-def material.  But it means that millions of HDTVs already sold that have only analog inputs could in the future become useless (or low-def) except for viewing whatever sitcoms or dramas the networks allow.  Some people think the FCC would forbid that for a while but not forever.


While all blu-ray disks are encrypted, the disabling of analog HD output is a separate flag, called the “Image Constraint Token” (ICT).  Thus far, all of the available movies have the token turned OFF, so analog HD output is allowed for these movies.  How long movies will be introduced with the token OFF is unknown.  The state of the token is supposed to be printed on the packaging.  (If the token is ON, the image on component video output will be down-converted to 540p.)


This author believes that Hollywood will not carry out its threat any time soon.  What Hollywood is most concerned about is movie piracy via the Internet.  That is currently not practical at high definition because it takes too long.  But if it should become practical and piracy proliferates, Hollywood wants to shut down those STBs that contribute to it.  (Hollywood would then not release films to distributors who enable STBs with piratable outputs.)  The FCC will certainly side with Hollywood if movie piracy makes movie making unprofitable.  This is not all bad since it guarantees home access to first-rate films. 


STB audio output options

An STB is likely to provide one or more of the following audio outputs:

  1. 6 channel audio  (6 wires with phono plugs)
  2. Coaxial digital audio  (1 wire with phono plug)
  3. Optical digital audio  (1 TOSlink fiber optic line)
  4. IEEE 1394 audio and video
  5. HDMI audio and video


Again, it is wise to plan this before buying.  You could be in a bind if the TV and STB do not have enough of the right connectors.


The HDMI Audio Problem

HDMI carries digital audio, but it is carried on the video signal lines during the video blanking periods.  This audio is in a form that is not easily convertible into SPDIF (optical or coaxial digital audio).  SPDIF is the normal input for A/V Receivers (audio power amplifiers).


This is not a problem if you are going to rely on the speakers or audio power amplifiers that come inside the TV.  It is also probably not a problem if your system has only one HDMI source.  But if you have more than one HDMI cable (multiple HDMI sources) then you might discover you have no way to connect up the A/V Receiver.


Here are your options:

1.    Just connect the TV SPDIF Output to the A/V Receiver.  Unfortunately many TVs lack this connector.  If your TV provides neither SPDIF nor “line” audio outputs then there is no way to connect it to an A/V Receiver.  Some TV SPDIF Outputs put out fewer than 6 audio channels.

2.    The A/V Receiver probably has multiple SPDIF inputs.  But if it does not have multiple HDMI inputs then you must use the TV to switch HDMI.  You will have to use two remotes every time you want to switch sources, or buy a universal remote and learn how to program it.

3.    Or you could buy an HDMI switch box that also switches SPDIF.  But most HDMI switch boxes do not have an SPDIF output.


More about DVI

DVI was originally developed for computer monitors, but has been adopted by HDTV.  DVI comes in different versions.  All versions use the same 29-pin connector.  Sometimes you can tell which version you have by seeing how many of the 29 pins are missing.


DVI-D is the version most commonly used for HDTV.  The five large pins are usually missing.  There is a single link version of this that uses only 18 of the 24 small pins.  Single link will work properly with all HDTVs. 


DVI-I uses all 29 pins.  The five large pins pass analog VGA signals.  Presently, DVI-I is used mainly by the computer industry, but front projector HDTVs by a number of makers support DVI-I.  There are DVI-to-VGA adapters and adapter cables available for these units.  Front projectors from a couple makers will accept component video signals through their DVI connectors.  These companies will provide DVI-to-Component adapter cables.  However this is nonstandard.


These adapter cables only work with DVI-I.  In most cases, if you want to connect a DVI unit to a VGA or component unit, these adapters will not work.  That would require a transcoder circuit that can convert between analog and digital signals.


HDMI is a single link DVI plus digital audio and a control line in a miniature connector.  It carries no analog signals.



The remainder of this page is nonessential reading.  You should skip it unless you are curious about how HDMI works.


DVI (and HDMI)

A CRT is a device with no memory.  DVI assumes all displays have no memory.  DVI assumes all displays require scanning in the manner of a CRT.  DVI will refresh each pixel once per frame, even if the pixel has not changed since the previous scan.


But instead of three analog signal lines (red, green, and blue) there are three digital signal lines (red, green, and blue) plus a clock line.  The clock line pulses once per pixel drawn.  The data signal rate is 10 times the clock rate.  During each pixel 10 bits are passed over each signal line, 30 bits in all.  24 of these bits are the color value.  The other 6 are control bits.  The maximum allowed clock rate is 165 MHz, which would allow 165 megapixels/second minus about 5% for vertical and horizontal blanking times.


The maximum data rate is said to be 1.65 Gbits/second, which would be the bit rate (data and control) over one digital signal line.  The three data lines together can pass 4.95 Gbits/second.


There are some variations on the above:

·         For 480i, pixels are sent multiple times to keep the data rate from being low.  The monitor discards the extra copies.

·         Data rates above 4.95 GB are allowed but require a “dual-link”.  A dual-link has 6 data lines, not 3.  If a dual-link is still not fast enough then the clock rate is allowed to exceed 165 MHz.


Each of the above clock and signal lines has 3 wires: plus, minus, and shield (a shielded twisted-pair).


It might be said that the industry chose a technology with an artificially high data rate to make piracy more difficult.


Two additional connector pins are used for DDC (Display Data Channel).  DDC is a medium-speed unidirectional channel between the two units.  The monitor uses it for identifying its operating modes and formats to the STB.



HDMI is partially compatible with DVI.  The new features with HDMI are:

·         HDMI single-link uses a miniature 19-pin connector.  An even smaller connector is available for portable equipment like camcorders.  Dual-link uses a miniature 29-pin connector

·         Digital audio is provided.  It is carried on the video data lines between frames and between lines.  (The timing is not the same as SPDIF.)

·         HDMI has one connector signal that DVI lacks:  One additional pin is used for CEC (Consumer Electronics Control).  This is a very low speed party-line wire that connects to possibly every STB and unit in the user’s system.  It conceivably allows any unit to control any other unit, possibly eliminating all but one remote controller.

·         Red, green, and blue are standard, but Y, Pr, and Pb are allowed.  Pr and Pb can be skipped every other pixel.


HDMI 1.3

·         The single-link clock speed limit is raised to 340 MHz.  Single-link will pass up to 10.2 Gbits/second (data and control).  Dual-link is used for speeds higher than that.  (This author has never seen the dual-link HDMI connector.  1080 60p will work with a single link, but 1080 120p would require a dual-link according to the HDMI 1.3 spec.)

·         Pixels with 8, 10, 12, and 16 bits/color are allowed.  There is always 1 clock pulse for every 30 bits.  So when the pixels are larger than 24 bits, the clock rate will be higher than the pixel rate.

·         An “Auto lip-sync” feature keeps the audio and video synchronized.

·         A “Reference Cable Equalizer” circuit makes longer cables possible.



This encryption scheme is used only for the cable between the STB and the monitor.  The MPEG data received from the source (DVD, satellite, or cable company) may be protected by some other encryption scheme.  If so it must be decrypted so it can be used for building the complete image in the STB’s memory, which is then re-encrypted using HDCP.  (Older DVDs are protected by an encryption scheme called “CSS”.  Blu-ray uses “AACS”.)


When encryption is enabled, every pixel data bit is exclusive-ORed with a pseudo-random data bit before transmission via HDMI.  The STB and the monitor agree upon the pseudo-random data pattern during authentication.  Non-video data is likewise protected.


Authentication is performed periodically, whenever the STB demands it.  Every box (STB and monitor) has a set of binary keys assigned to it.  These keys are generated by a company called Digital Content Protection LLC, a subsidiary of Intel, and are assigned to the box when it is manufactured.  Some of the steps in authentication are:

1.    The public key – private key method is used to prove that each box is an authorized unit.  The public key is 40 bits long and the private key is 2240 bits.

2.    The Key Revocation List is searched to see if a box has been associated with piracy.  The DVD or distribution company provides this list.

3.    A seed value is chosen for the pseudo-random data stream.


The encryption scheme is designed to be hard to crack.  Breaking it probably requires saving the entire movie on disk and then launching a deciphering program that could take many hours or days.  But the high data rate of HDMI means that 30 terabits of disk memory would be required to save the entire encrypted data stream of a two hour 1080p movie, and not many people have that much storage at their disposal.


Stealing a valid set of keys is easier.  “HDCP Stripper” boxes with stolen keys can be purchased.  Such boxes are illegal in the United States, and their keys could someday end up on the Key Revocation Lists.


Some manufacturers have used the same set of keys in all their boxes of the same type.  If any of those boxes ever ended up on the Key Revocation Lists then many innocent people would lose functionality.



IEEE 1394 has obvious advantages over HDMI.  The lower data rate makes it easier to route the signal around the house, either through CAT5-like cable or wirelessly.  The MPEG decoding can be more tailored to the capabilities of the monitor if the monitor does the decoding.  This usually results in a better picture, easier operation, lower cost, and a much simpler planning process for the buyer.


But, according to rumors, the DTCP encryption scheme used by IEEE 1394 has been successfully violated, and a program for doing so might be easily downloadable.  Hopefully someday the studios will come to their senses and adopt an un-crackable successor to DTCP.








This page is part of “An HDTV Primer”, which starts at    www.hdtvprimer.com